The report/releases I've been reading from the majors suggest that they expect to see a long term price in the 700's with short term spikes into the low $1000's.  I've also seen much higher price projections but I expect that the majors will be conservative in the numbers they use so I used $750.  All the input numbers I used were 'guesstimates' based upon what I think might happen.  

As such the calculation I did was rough.  I used average prices over 20 years in the NPV to give an idea of how prices and IRR's relate to what a major major will pay.   The majors would use monthly or quarterly prices depending on what they had for information to get the most accurate NPV they could.   If their intent was to hedge their production, input costs, currency exposure, etc they would use their hedged values.  In the end they too are guesstimating but with a much more detailed calculation using I hope much better information.

If I sub in 20 year average prices of $800, $900, $1000, $1100, $1200 and $1300 at 10% IRR you get roughly $249/oz @$800, $291/oz @$900, $348/oz @ $1000, $397/oz @$1100, $446/oz @$1200 and $496/oz @$1300.    About $50 more per ounce for every $100 the 20 year average price increases.

As I mentioned these are rough calculations.  In addition to the IRR requirement, the timing of revenue and costs also impacts the NPV.   Hence the rate of mining and life of the mine have a big impact on the calulation.  Hhigher mining rates over a shorter time period yields higher NPVs then lower rate for a longer time assuming everything else stays the same.

At the same time lack of information about royalties amounts, corporate tax rates, windfall taxes, social support obligations etc add signifcant uncertainty to the model outcomes.   Any cost that results in a lower than expected cost yields a higher NPV and vice versa.   They will discount what they pay us to take into account the uncertainty so in the end the more certainty there is with regard to mining laws, taxaton, etc the better off we are. 

.... Been There